(a) Field of the Invention
The present invention relates to a six-speed powertrain of an automatic transmission for a vehicle enabling six forward speeds and one reverse speed.
(b) Description of the Related Art
A multi-stage gearshift mechanism of an automatic transmission includes a plurality of planetary gear sets. A powertrain having such a plurality of planetary gear sets varies the torque in multi-stages and outputs it to an output shaft when receiving a converted engine torque from a torque converter.
The more speeds the powertrain of an automatic transmission has, the better the power performance and fuel consumption. Therefore, it is desirable for powertrains to have as many speeds as possible.
Even for the same number of speeds, durability, power transmission efficiency, and size/weight of a transmission are substantially dependent on how planetary gear sets are arranged. Therefore, research for more structural strength, less power loss, and more compact packaging are continuously being conducted.
Usually, development of a powertrain using planetary gear sets does not devise a wholly new type of planetary gear set. To the contrary, it involves how single/double pinion planetary gear sets or a Ravingneaux planetary gearset are combined, and how clutches, brakes, and one-way clutches are disposed to the combination of planetary gear sets such that required shift speeds and speed ratios are realized with minimal power loss.
For a manual transmission, too many speeds cause a driver the inconvenience of excessive manual shifting. However, for an automatic transmission, a transmission control unit automatically executes shifting by controlling the operation of the power train, and therefore, more speeds usually implies more merits.
Accordingly, research of four-speed and five-speed powertrains has been undertaken, and recently, a powertrain of an automatic transmission enabling six forward speeds and one reverse speed has been developed. However, existing six speed automatic transmissions can have some drawbacks. For example, in one such powertrain, the first, second, and third ring gears are respectively connected with the second, third, and first planet carriers. Therefore, respective planet carriers are required to be separated, and such a configuration causes drawbacks of lengthening an automatic transmission and increasing a weight of the transmission. In addition, the ring gears are not interconnected and in this case, structural strength may not be sufficiently strong. Also, such a powertrain shows low gear ratios at the reverse gear and the highest gear such that gradeability in the reverse speed and front wheel drive performance may become inferior.
Furthermore, the gear ratio at the first forward speed can be too high and accordingly, the tire may easily slip at a hard acceleration. In addition, a step ratio between the second and third forward speeds is large, and thus tip-in shock may become large during a city driving circumstances of an intermediate speed.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.